Williams, 29, was the No. 5 overall pick in the 2005 draft by the Bucs. He rushed for 1,178 yards as a rookie but missed most of the 2007 and 2008 seasons with knee injuries, and he lost his spot as Tampa Bay's featured back last season to LeGarrette Blount.
Williams is expected to be the No. 2 man in the Rams' backfield behind Jackson. Norwood, who also returns kicks, is expected to be No. 3.